Sound Deadening for Wood Floors
November 6, 2008
Noise transfer between spaces has been an issue with hard surface wood flooring for many years. New York City was one of the first areas to make specific requirements for sound transfer. These standards were most often applied to multistory apartments. The standard often referred to is Sound Transmission Class (STC) of 50 and Impact Insulation Class (IIC) of 51. STC is the transfer of sound generated within a space through the wall, floor, or ceiling to an adjacent space. IIC is the sound that is generated by a tapping machine directly on to the structure (wall, floor, or ceiling) and sound transferred through the structure to the adjoining space .
NOFMA conducted sound transmission tests in 1982 to address those systems that did or did not meet the standards. The basic systems used were all floating systems of some variety. Woven nylon matting, 1/8” and 1/4” closed cell foam, and asphalt insulation board were used as the sound deadening material. Both wood joists systems and concrete slab systems were tested. Wood joist systems of framing with 1/2” plywood subflooring, sound material, 3/4” floating subfloor, and 3/4” strip flooring; all failed the required standard. The concrete slab system using 1/2” insulation board, 3/4” floated plywood subflooring, and 3/4” strip flooring nailed with 1 1/2” cleats; was the only system that met the standard with STC of 52 and IIC of 55. The slab system using the nylon matting was modified with a suspended ceiling below of 5/8’ gypsum in metal channels held by 9” wires and 3 1/2” fiberglass insulation on the gypsum board also met the standard. This system produced STC of 60 and IIC of 61.
A review of these systems all pointed to isolating the flooring and subflooring from the primary support system to obtain the highest readings. Metal fasteners all transfer noises effectively, so nailing flooring to a subfloor which is also nailed to the slab or framed system will be a noisy system. Items and procedures that will interrupt some of the noise transfer are: floating the flooring subfloor; placing a sound deadening material under the subfloor; isolating the edge of the flooring system from the surrounding structure; gluing the different components together; and/or suspending the underlying ceiling.
The question is often asked for a nail down flooring system; “Can I put the sound deadening material directly under the flooring and nail through it into the subfloor using longer fasteners?” NOFMA does not recommend this procedure since the sound material will not hold the flooring fastener properly and most materials compress or deflect with point loading. Flooring noises, creaks and squeaks, and movement are the likely result.
The basic directive for effective sound deadening and 3/4” thick solid wood flooring is to build the subflooring and flooring independently from the support system and sound deadening material. Follow the manufacturer’s recommendation for installation of the acoustical material. Generally, for a suspended slab, place or glue the acoustical material to the slab. Next place (float) or glue the subflooring over the acoustical material, add the #15 felt and nail the flooring so fasteners do not reach the acoustical material. In order to confirm the STC and IIC are met, it may be necessary to perform a field test by a qualified Acoustical Engineer. The manufacturer of the acoustical material may also confirm their material meets the standard for the specific system.
Some other particulars that may affect and increase sound transmission are the materials that are in contact with the flooring. Around the perimeter be sure to isolate the flooring system edge from the wall lines. Adding insulation and/or caulking in the perimeter expansion gap can facilitate this. Also, to maintain isolation, the base and shoe mold should not touch the surface of the flooring. If the acoustical standard is not met with all these precautions, suspending the ceiling in the space below can help reduce the noise transfer.
The sound standard may be obtained easier by direct glue down method of either an engineered or solid wood product where the acoustical material is glued to the substrate and the flooring glued to the acoustical material. In this case the manufacturers of the acoustical material, the adhesive, and the flooring must confirm their product will perform as intended. The acoustical layer must be able to withstand normal seasonal changes and associated wood movement of the geographical area. This is especially important where direct glued solid wood is involved.
Other alternative flooring materials are the floating engineered flooring products. Either the click-and-lock systems or the edge-glued systems. These are generally placed over a foam pad and are effective sound deadeners. Most of the manufacturers of these systems can supply documentation of sound transmission testing.
Sleeper systems can also meet the requirement. But, the sleepers must also be isolated by gluing or floating over an acoustical material. Additional dampening of the sound can be obtained by filling the sleeper space with insulation. An elaborate sleeper system as reviewed by HUD in 1971 consisted of: 8” joists 16” O.C. with 3 1/2” insulation stapled between; subflooring 1/2” plywood glued and nailed; 1/2” fiber board stapled 24 “ O.C. to subflooring; 2” x 3” sleepers glued 8” O.C. to the fiber board; and 25/32” thick flooring nailed to sleepers. On the ceiling side of this system resilient channels were placed 24” O.C.; screwed perpendicular to joists; 5/8” gypsum wallboard screwed each 12” along the channels; taped and finished and the entire periphery caulked and sealed. This system tested at STC = 52 and IIC = 51. This system would likely be cost prohibitive in today’s market and newer materials could likely be substituted to meet similar readings.
There are many acoustical materials available to deaden the sound transmission. Chose one that will perform with the system you choose. Check with the different manufacturers for compatibility of the different parts. Ask for testing data to confirm the standard is met. Check out the NOFMA web site www.nofma.org and review the floating systems associated with radiant heating systems, which can be a guide for floating a system over acoustical materials.